How to Rid Your Cannabis Plant of Powdery Mildew

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In Common Marijuana Insects and Pests to Look Out For we mentioned that powdery mildew is one of the most common problems that can plague your marijuana. This disease, which is caused by fungal species in the genera Erysiphe, Microsphaera, Phyllactinia, Podosphaera, Sphaerotheca, and Uncinula, is fairly manageable in its initial stages but it can also do quick work of your plant and make it unusable. 

Powdery mildew is a blanket term for several kinds of fungi that can cover your plant in fine, dust-like spores. The bad thing about it is that it’s airborne, there is a good chance that you can carry it to your plants. It can also create a network on your plants before it even becomes visible. The good thing is that proper preventive methods will greatly minimize the chance of your plants getting it and it’s also unlikely that you have to resort to pesticides or harsh chemicals to get rid of it. There are also certain strains that are more resistant to it.   

How to identify powdery mildew

Detail of powdery mildew, plant disease

Powdery mildew will look like a fine layer of white or gray dust on your plant’s leaves. Don’t attempt to dust this fuzzy, flour-like substance off, that will just kick up the mildew and spread it even more.  

However, that visible “dust” is not all the mildew on your plant, there’s actually more than you are able to see. Cannabis growers are still uncertain whether powdery mildew is something that only attacks the surface of the plant or does it spread throughout the plant even on the inside, so if it gets to your buds, you should throw them out.

Why is powdery mildew bad

Don’t be tempted to keep any of the buds that came from a plant that caught powdery mold during its flowering stage. Even if you can’t see it, your buds probably already have it and if you use it, it’ll make you sick. Breathing in powdery mildew can cause respiratory infections like aspergillosis, so you should also take great care not to breathe it in when removing mildew-ridden leaves and plants. 

It is important to stop powdery mildew during the vegetative stage. During that time, a bad infestation can stunt the growth of your plants. Since powdery mildew can be transmitted by practically anything: your clothes, pruning shears, a simple breeze, containing it can be difficult. While there are ways to salvage plants infected during the flowering stage, smoking buds that have gotten mildew carries significant health risks, it’s better to just prune them off and dispose of them.  

When you might notice powdery mildew

Powdery mildew in its initial stages

Your plants can get powdery mildew at any stage of its life. It’s easy enough to manage during the growth stage but it can easily ruin your harvest during the flowering stage. 

Powdery mildew is unmistakable, it will look like somebody dusted your plants with flour. In some cases, it may also look like fine grey house dust. New growers may mistake these spores for THC-filled trichomes but under magnification, the two look nothing alike. To the naked eye, the start of a powdery mildew attack appears like patches of white powder on your plant’s leaves. Under a magnifying glass, it looks like a network of clumps which, under the microscope, look like chains standing up.

Organic methods to deal with powdery mildew

Growers are still divided on whether powdery mildew is systemic or superficial. This is why you may encounter a number of recommended treatments for it that may seem contradictory. However, the fact remains that powdery mildew can still be present even if you can’t see it anymore. Below are some of the most popular natural treatments that growers have had significant success with.

  • Preventive measures – As we’ve already mentioned in past guides, a well-maintained growing environment will be inhospitable to powdery mold and other cannabis pests. This disease thrives in warmer temperatures, higher humidity with little to no air circulation, so make sure that in addition to your fans, you also have a humidifier or dehumidifier that can maintain the right relative humidity in your grow room. You should also invest in an air purifier if you can afford it since it will help you deal with other airborne diseases.
  • Ultraviolet light – UV-C radiation is used to sterilize surfaces and water of bacteria and viruses for years. It is powerful enough to kill powdery mildew and even mites on exposure. However, it is also hazardous to all other living things, it can damage your plant’s DNA as well as your eyes and skin. Furthermore, not all UVC lamps are suitable for killing mold and mildew on cannabis. Because of this, some growers think that using UVC for disease control on cannabis is just not worth it. 

  • Hydrogen peroxide and horticultural oils – Most growers swear by these two as they seem to be the cheapest, easiest, lowest-risk solution for addressing powdery mold. Both methods work by preventing spores from multiplying. A 1:3 mixture of hydrogen peroxide and water is the recommended starting point since a stronger solution could burn your plants. Meanwhile, only use horticultural oils if your grow room temperature is between 40-90° F with a low RH so that the oils can evaporate quickly and not “cook” your plant. Some examples are The Amazing Doctor Zymes Eliminator Concentrate.

  • Bicarbonates (Baking soda) – Most growers have also found success using bicarbonates like potassium bicarbonate and baking soda which increases the pH of the leaf surface, inhibiting fungal growth/spore germination. The recommended ratio is one tablespoon of baking soda per quart of water mixed with a spreader like vegetable oil or non-antibiotic dish soap.
  • Milk – As crazy as it sounds, growers around the world have discovered that regular cow’s milk can prevent the spread of powdery mildew. Scientists aren’t exactly sure at present why it is effective, but studies seem to point that milk acts as a natural germicide and powdery mildew seems to be sensitive to its naturally occurring salts and amino acids. However, it may leave an undesirable odor on your plants after a few days.

Non-organic methods to deal with powdery mildew

The organic and natural methods mentioned above are fairly adequate when dealing with powdery mildew in a home growing setup. However, these won’t be effective in addressing a late stage infestation or an outbreak in a commercial growing setup. Growers who are in danger of losing lots of plants and are desperate to kill powdery mold fast often resort to synthetic fungicides and other non-organic methods such as the ones below:

  • Myclobutanil (Eagle 20, Nova)
  • Copper Octanoate
  • Chlorothalonil
  • Thiophanate-methyl
  • Propiconazole

Frequently Asked Questions on Powdery Mildew

How fast does powdery mildew reproduce?

Your plant may already have powdery mildew spores for 4 to 7 days but it won’t be visibly noticeable for another 1 to 2 days.

What are signs that you have a powdery mildew infestation?

Powdery mildew will look like a fine layer of white or gray dust on your plant’s leaves. Don’t attempt to dust this fuzzy, flour-like substance off, that will just kick up the mildew and spread it even more.  

Is powdery mildew harmful to cannabis plants?

Powdery mildew can significantly impair the functions of your plant. However, the worst thing it can do is infect your buds, making them unusable.


It cannot be stressed enough that preventive measures are key when dealing with invisible, airborne diseases like powdery mildew. While powdery mold is hardly fatal, smoking weed that has been significantly infected could give you a bad allergy or respiratory problem, so it’s a good idea to wash your marijuana plants right after harvest just to be doubly sure. You can even use hydrogen peroxide for good measure. When in doubt, check your marijuana before smoking it. Even if it looks ok, there;s still a possibility it has gone bad inside, so smell it and open it up. If it’s soft or moist on the inside and has a bad smell, throw it out immediately and check the other buds in the batch as well.

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